1. Technical Field
This invention relates generally to a gaseous polarization process, and in particular to a method and system for producing polarized .sup.129 Xe gas in a commercially feasible manner.
2. Discussion
Xenon gas, in addition to its present use as a safe general anesthetic, may have a variety of other potential medical applications. When ingested by a subject, xenon is transported from the lungs to the blood and on to the rest of the body, and concentrates in lipid and protein tissues such as those forming the lungs and the brain. Because xenon concentrates in these particular biological environments, the gas nuclear spin 1/2 isotope, xenon-129 (.sup.129 Xe), when polarized, produces beneficial results when used in magnetic resonance imaging (MRI) applications.
In MRI applications, magnetic resonance signals are produced by weakly polarized proton spins found within biological tissues, with the vast majority of proton spins residing in water molecules. These proton spins are weakly polarized by large magnetic fields: polarizations of approximately 10.sup.-5 in magnetic fields of several tesla are typical. Because the strength of the polarization signal is in direct proportion to the strength of the MRI signal, it is desirable to have a stronger polarization of the protons in the tissues that are being imaged. However, in certain biological environments, such as the lung gas space and lipid membranes in the brain, there is very little water present. Thus, poor quality proton MRI signals are typically produced in these environments.
Highly polarized .sup.129 Xe gas, when inhaled by or injected into a subject and absorbed by biological tissues, creates higher resolution images of these tissues and other weakly polarized MRI environments. Such beneficial results have been experimentally shown as reported by M. S. Albert, G. D. Cates, D. Driehuys, W. Happer, B. Saam, C. S. Springer, Jr., and A. Wishnia in an article entitled "Biological Magnetic Resonance Imaging Using Laser-Polarized .sup.129 Xe", 370 Nature 199 (21 Jul. 1994), hereby incorporated by reference. To date, however, presently-known methods of producing polarized .sup.129 Xe gas have proven to be cost prohibitive from a commercial production standpoint.
Known methods of polarizing xenon gas incorporate a resonant light source to optically pump an alkali metal vapor to transfer the angular momentum of photons from the light source to atoms of the alkali metal vapor via cyclical resonant absorption or scattering. As alkali metal vapor atoms absorb this angular momentum, the non-polarized xenon gas is introduced into the same environment as the optically pumped alkali metal atoms. The optically pumped alkali metal vapor atoms then collide with the non-polarized xenon atoms, thus transferring polarization from the alkali metal vapor atoms to the xenon atoms. These collisions thus polarize certain isotopes including .sup.129 Xe.
Several types of light sources are potentially suitable for optically pumping the alkali vapor. These light sources include: alkali lamps; dye lasers; Ti-sapphire lasers pumped by argon ion lasers; and single mode diode lasers. As discussed below, all of these light sources have significant practical limitations that make them unsuitable for polarizing .sup.129 Xe gas in a commercially viable system.
Lamps are the oldest technology and provide a low power, incoherent light (less than 1 milliwatt) at a modest cost. However, these light sources are not capable of the high power optical pumping required for efficient production of spin polarized .sup.129 Xe gas.
Dye lasers provide a much higher power light than do the aforementioned lamps (up to approximately 1 watt). However, dye lasers are relatively expensive, large, complex, delicate and have high associated maintenance costs. Thus, these lasers are not desirable for the economical production of polarized .sup.129 Xe gas.
Ti-sapphire laser systems are desirable for optical pumping as they can provide up to 5 watts of narrow spectrum light with a spectral width of less than 30 GHz. However, Ti-sapphire laser systems suffer similar impracticalities associated with dye lasers: the laser systems are delicate, large in size, have associated high maintenance costs and are expensive (approximately $100,000 for a complete Ti-sapphire laser system).
Single mode diode lasers are desirable for optical pumping as they are small, robust and relatively inexpensive. However, these lasers provide less than 200 milliwatts of narrow spectrum light (i.e., a spectral width of less than 10 GHz). Because of their low power, single mode diode lasers can not efficiently polarize large quantities of .sup.129 Xe gas.
Another commercially available light source is the diode laser array. Diode laser arrays are desirable because they are small in size, robust, portable, easy to operate and maintain, and have relatively inexpensive initial cost (approximately $10,000 for a complete laser system). Typical diode laser arrays can provide tens of watts of power over a spectral width of about 1000 GHz at the alkali vapor optical pumping transition, and are solid state devices. Diode laser arrays to date have not been thought to be practical for optically pumping an alkali metal vapor in a .sup.129 Xe polarization process because of their large spectral width.
Diode laser arrays have been used to polarize .sup.3 He, as isotope of helium, as set forth by M. E. Wagshul and T. E. Chupp, in an article entitled "Optical Pumping Of High Density Rb With A Broadband Dye Laser And GaAlAs Diode Laser Arrays: Application To .sup.3 He Polarization," 40 Physical Review 4447 (1989) which is hereby incorporated by reference. However, the process of polarizing .sup.3 He and .sup.129 Xe gases differ significantly due to the different elemental properties associated with each gas, including atomic masses (the ratio of the atomic mass of helium versus the atomic mass of xenon is approximately 1:43), atomic size, nuclear size and atomic chemistry. These differences are manifested in quantitive and qualitative differences. For instance, the rate of alkali to .sup.129 Xe polarization transfer is hundreds of times larger than the rate of alkali to .sup.3 He polarization transfer. Also, alkali-.sup.129 Xe van der Waals molecules can form in a polarization chamber, but alkali-.sup.3 He molecules cannot. Further, special materials (e.g. aluminosilicate glass) or special treatments must be used to construct a .sup.3 He polarization chamber because the very small .sup.3 He atom can diffuse into the walls of many common materials (e.g. Pyrex glass) and quickly be de-polarized. The much larger .sup.129 Xe atom does not have this diffusion problem. In addition, because .sup.129 Xe is much more chemically active than .sup.3 He, a .sup.129 Xe polarization chamber's inner walls must generally be coated with a special material (e.g. octadecyltrichlorosilane) to prevent de-polarization. No special coating is needed for .sub.3 He. Finally, the polarization of the .sup.3 He and .sup.129 Xe gas require significantly different operating temperatures, alkali vapor pressures, different polarization chamber construction materials and different optical intensities. Because of these differences, commercially available diode laser arrays have not been recognized as a viable alternative for a xenon gas polarization system. Nevertheless, as discussed below, it has been discovered that diode laser arrays are effective in polarizing .sup.129 Xe gas by optically pumping an alkali metal vapor.
Therefore, what is needed is a polarization system incorporating a high power light source such as a diode laser array that is small, robust, and relatively inexpensive, to optically pump an alkali metal vapor to produce polarized .sup.129 Xe gas in a cost-effective and heretofore unachieved manner.